One prior system for measuring the neutron spectrum of a beam of neutrons is the use of activation foils. Unfortunately, activation foils can only measure neutron flux at specific, discrete energies. Therefore, in order to obtain an energy spectrum, numerous foils are necessary. In addition, there is no "real-time" output of spectral information where such activation foils are used.
Another method of determining the energy spectrum of a beam of neutrons is time-of-flight measurements. Systems of this type require sophisticated and expensive equipment, and must use a pulsed neutron source and time-of-flight electronics. Such systems are not usable with passive sources emitting a time-continuous source of neutrons.
Disclosed in U.S. Pat. No. 4,180,736 (Goodman) is a time-of-flight measuring system for detecting neutron energies. The system includes a scintillator positioned at a desired angle with respect to the incoming neutron beam such that the sum of the transit times of neutrons and photons in the scintillator is substantially independent of the position of the scintillation points within the scintillator. Unfortunately, this system must use pulsed neutrons and is not able to determine the neutron energy of slow (less than 10 KeV) neutrons.
In U.S. Pat. No. 4,291,227 (Caldwell et al), a non-destructive method for distinguishing among and quantifying the mass of individual fuel plates in fuel drawers is disclosed. The method includes a neutron counter coupled to a gamma-ray spectrometer. The data is analyzed by a computer.